Re: Fo shift issues with spark C loading? was, Tesla Coil Blunderbusses

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

John (Freau, & all)-

Ken Herrick's comments interspersed:

On Sat, 14 Apr 2001 19:55:24 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" 
> <FutureT-at-aol-dot-com>
> 
> Malcolm, Ed, Ken, all,
> 
> Some have questioned whether the spark's capacitive loading shifts
> the coil's required primary resonant frequency.
> 
> For max spark length at low power
> (27" spark), I had to tune my coil at 19 turns.  For max spark
> length at high power (42" spark), I had to tune at 21 turns.
> Clearly, something is affecting the required tune point at 
> different power levels and spark lengths.  Interestingly, this
> degree of frequency shift based on my tap point position
> agrees with the assumption of 1pF per foot of spark length,
> which Terry has suggested.  I think Greg Leyh found a similar
> figure for the spark's capacitance.  I forget how they determined
> the spark's capacitance.  Was it by back calc'ing the frequency,
> or was it derived some other way?
> 
> I remember that Ken H said that his frequency did not shift
> when his spark broke out of the toroid.  Considering the large
> size of his coil and toroid, the frequency shift would be only
> about 2% or so.  I'm assuming that his measurement method
> would clearly show whether a 2% frequency shift had occured?

See my posting on 4/6:  I measured ~1.2% difference for the secondary's
Fr before vs. after spark break-out.  That was pretty accurate since my
scope images were reasonably stable.  Keep in mind that my primary is
untuned.
> 
> I'm trying to create an explanation in my mind that would 
> satisfy both observations above.  If we accept that Ken's
> frequency did not shift with spark breakout, then the only two
> explanations I can think of are:  (1) the presence of the
> ion cloud around the toroid also shifts the frequency.  Thus,
> his frequency would have already shifted due to this cloud
> before breakout.  But if this is the case, then as the coil
> starts up, the frequency should shift as the ion cloud is
> formed and builds up.  I don't know if Ken checked for this
> effect?  

I've looked again as carefully as I can at the Fr period at the very
beginning of voltage build-up vs. just before the spark.  Unhappily,
there's some jitter, spark to spark, either when I trigger from the
voltage itself or from my gating pulse although the waveform is fine: a
smooth sine wave.  But I can see no more difference than 6.8 vs. 7.2 us,
with the longer period near the break-out time.  That's ~5.7%.  I think
that the 1.2% measurement I had made before was for just prior to
break-out vs. well after.  So perhaps the e-field effects just prior to
breakout are causing the ~6% difference during the voltage build-up.  My
spark rate for this test was quite low: just ~2/sec.

But now:  Here's what I've just done today in order to try to alleviate
the jitter problem:

1.  Sync the scope on its signal input, that being the secondary's
e-field picked up by a probe.  Set it for dc triggering.

2.  Set the sweep for 2 us/cm.

3.  Set my s.s. coil to its interrupted-burst mode.  There, I get 7 pulse
bursts in sequence, each one 64 cycles long (at the instantaneous
secondary Fr), for each sparking event.  I set the rate of events to
perhaps 2/sec.

4.  Adjust the trigger level for stable triggering at some convenient dc
level of the picked-up waveform (which is a sine wave).

5.  On the screen, I see 7 sweeps occurring during each sparking
event-time; the sine-wave images overlap, with all of them starting at
the same dc level of each wave.  One of those 7 sweeps incorporates the 7
cycles following the trigger-level point of the first burst, which is the
one occurring prior to the first spark break-out of the sparking event.

6.  This is what I find:  In the course of 7 cycles, across the screen, I
find a maximum phase shift of 180 deg accumulating amongst the 7
superimposed images.  That means that the frequencies of Fr differ, over
the course of the ~7 ms time period during which I emit 7 "mini-sparks",
such that a phase-shift of 180 deg. occurs over the span of 7 cycles of
spark voltage.

Now, that's not very much Fr-shift.  I haven't yet figured out just how
much but perhaps those more analytical could do that in a trice.  Also, I
can't yet tell exactly which ones of the 7, 7-cycle-long images has which
period--but perhaps that's immaterial to the discussion.

Does that seem like a fairly definitive test?

(2) either the sparks do not have any capacitance,
> or if they do, it's not shifting the frequency.  I don't see why
> the sparks would not have capacitance though, or why it
> wouldn't shift the frequency if it does.

It's just conjecture since I'm no expert, but I'd think the capacitance
of a skinny little spark would be negligible compared to that of a big
fat toroid.  (For the uninitiated, those are technical terms of the art
that I employ.)

  If the ion cloud does
> not cause a frequency shift, and if the sparks do not cause 
> a frequency shift, then what could be creating the need for 
> me to retune my coil from 19 turns to 21 turns as the sparks
> get longer?  (For each power level, I retuned for longest sparks.)

Good question, indeed.  Since I ain't got such a primary, perhaps I can't
answer it.
> 
> It is true that the primary may need to be tuned lower in
> frequency than the secondary for best power transfer.  But if
> this is the only reason to tune lower, why would the best tune
> point vary with power input and spark length?
> 
> My guess is that the primary needs to be tuned lower in frequency
> for two reasons; to set it to the lower split response, and to 
> compensate
> for capacitive spark loading (or capacitive ion cloud loading).
> 
> I do see what Malcolm is saying about the spark breaking out after
> most of the energy has been transfered to the secondary.  This 
> would suggest that the best tune point should not vary with spark
> length, yet in my tests, it did.  It is possible maybe that the ion
> cloud C loading adds just as much capacitance as the streamers?
> (This ion cloud persists between bangs and may affect the needed
> tune point for the primary.)  Alternatively, maybe the sparks break
> out sooner once a lot of ionization has built up along the streamer
> paths.  This could explain why the primary needs to be retuned.
> Still, if it's the sparks that have the greatest freq shift effect
> due to their capacitance, then Ken should have seen a freq shift
> when his sparks broke out.

It might be either a lot of ionization (tho I rather doubt that that
hangs around very long) or, as I'd think more likely, a lot of
residual-heat air-paths leading away from the toroid.  But still, without
an "ion cloud", I wouldn't think there'd be much added capacitance--just
due to the heat.
> 
> Malcolm suggested that the lower primary tune point may make
> it harder for the energy to return to the primary.

How far apart can those dual peaks be?  I shouldn't think that the
coupling characteristics would change that much over that relatively
small change in frequency.

  Certainly if 
> the
> lower tune point makes the sparks longer, (due to better energy
> transfer), the sparks will burn up more energy and leave less left
> to return to the primary.  Is there any other mechanism at work?
> 
> If this is the case, then a test could be done using no breakout 
> from
> the toroid.  The coil would be tuned for max voltage or field 
> strength
> from the toroid, at a low power level.  Then the power level would
> be increased, and the coil would be retuned if needed for max 
> voltage
> or field strength.  If the coil needs to be retuned, this would 
> suggest
> that the ion cloud C is affecting the needed tune point even 
> without
> streamers.   I'm not sure if this is a perfect test though.  Does
> anyone have any other comments or insight into these issues?
> Am I missing some point?
> 
> Cheers,
> John Freau

There's one thing I have noticed that may have some bearing:  When I
reduced my primary's dc supply voltage so that I just did not get
break-out from the toroid, I recall that the mains input power required
was significantly greater.  Once I raised the voltage high enough to
spark, that power dropped.  Curious, hmmm?

Ken Herrick
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